This invention relates in general to the drying of granular products and more particularly to an apparatus and process for drying granular products with microwave energy.
Many granular products, as orginally produced, contain an excessive amount of moisture and cannot be stored or used in that condition. Consequently, the product, whatever it may be, must be dried. Seeds such as corn, rice, and soybeans are examples of such products.
Practically all of this nation's corn, soybean, and rice harvest is currently dried in hot air dryers which consume enormous amounts of fuel and produce a considerable amount of dust. Furthermore, these dryers often raise the temperature of the seed so high that the seed is incapable of germinating, and this of course renders the product useless for planting. Even when the seed is destined solely for feeding purposes, the excessively high temperatures diminish its nutritional value, harden its shells, detract from its taste, and impair its ability to withstand long periods of storage.
Some granular products can be successfully dried with microwave energy, and the dryers which employ this form of energy usually take the form of a belt that moves the granular product through a microwave field. These dryers are useful where a high power density can be applied to the product without damage to the product, but most seeds do not fall into ths category. On the contrary, high power densities raise the temperature of the seed to excessively high values, creating all of the problems heretofore mentioned.
U.S. Pat. No. 4,015,341 discloses a seed drying apparatus that employs microwave energy for drying various seeds, but the seeds are maintained under a vacuum while being subjected to the energy. In this manner the moisture is removed at much lower temperatures. However, the critical value at which a microwave field breaks down diminishes with pressure, so that arcing and corona discharge are more likely to occur under vacuum conditions. Hence field strengths must be maintained relatively lower than would otherwise be acceptable at atmospheric pressure. Also, the apparatus of U.S. Pat. No. 4,015,341 is quite massive in relation to its productivity.